The present invention relates to an audio amplifier. In particular, it is suitable for use for a digital amplifier (class-D amplifier), which drives a speaker through switching operations of MOS transistors.
The conventional class-A/class-AB amplifiers are known as analog amplifiers. On the other hand, the class-D amplifier is known as a digital amplifier, because such amplifier causes power MOSFET to perform switching operations and causes a speaker to be driven. The power efficiency of the digital amplifier is better than that of the analog amplifiers. Thus, with the backdrop of a demand for miniaturization and low consumption of power for the audio equipment in the recent years, there has been an increase in audio equipment that adopts a digital amplifier.
FIG. 1 is a diagram showing a part of the structures of a conventional digital amplifier. Here, a so-called 1-bit digital amplifier is shown. Differently from the PCM method, which records absolute quantities of quantized data for all sample points, the 1-bit method only records variations from the immediately preceding data as a binary data, and neither thinning out data nor interpolating data is performed as in the PCM method. Thus, a 1-bit signal obtained from quantization shows characteristics quite similar to those of analog. Therefore, a D/A converter is not required, and original analog signals can be reproduced by a simple process which digital signals with high frequency components are removed by the low pass filter arranged at the final stage.
In FIG. 1, “10” denotes an IC chip. Such IC chip 10 integrates the power switch 1, which is composed of a full bridge structure of pMOS transistors Q1, Q2 and nMOS transistors Q3, Q4. pMOS transistors Q1, Q2 are connected to the power source voltage (VDD) outside the chip via the terminal 4. nMOS transistors Q3, Q4 are grounded outside the chip via the terminal 5.
Also, although the illustration is omitted, the IC chip 10 integrates the circuits so as to drive all MOS transistors Q1 to Q4 of the power switch 1. The circuits for such driving include the circuits that perform Delta-sigma Modulation or PWM (Pulse Width Modulation) for input audio signals, and that generate a driving signal having a pulse width in accordance with such audio signal.
All MOS transistors Q1 to Q4 of the power switch 1 perform switching operations based on the driving signal generated by circuits not illustrated in the FIG. That is to say, according to the pulse width of the driving signal, the time in which all MOS transistors Q1 to Q4 are ON is controlled. Through this, the power switch 1 amplifies the audio signal based on the power source voltage VDD for an amount equal to the controlled driving time, and outputs the amplified audio signal.
The audio signal amplified by such power switch 1 is outputted outside the IC chip 10 via the terminal 6 and 7. And such audio signal becomes an analog audio signal through the LPF 2, which is composed of the coils L1, L2, and condenser C, and is outputted from the speaker 3.
Regarding the audio amplifier structured above, in order to obtain large output power (for example, greater than or equal to 10 [W]) in the speaker 3, the power source voltage VDD supplied to the power switch 1 is required to be high. However, in order for this to occur, the voltage tolerant of elements within the IC chip 10 must be sufficiently high. In order to achieve a higher voltage tolerant of elements, a special device must be made in the product process of the Chip 10, and equipment exclusively used for this purpose is required. Thus, there has been a problem in that such matters could not be realized easily.
Moreover, various circuits, in addition to the power switch 1, are integrated within the IC chip 10, and there exist some circuits from among them that operate by low voltage. Thus, when the power source voltage VDD to the power switch 1 becomes higher, the circuits that operate by high voltage and the circuits that operate by low voltage are combined within the IC chip 10. In such cases, a complicated control circuit that combines a control system with high voltage and a control system with low voltage, including a level-shift function, is required. However, the process of combining such control systems with high and low voltages cannot be easily realized. Additionally, the fact that the structure of the IC chip 10 becomes more complicated and larger has been problematic.
The purpose of the present invention is to resolve such problems, and to obtain large output power for the speaker while the power source voltage to the power switch (IC chip) remains low.